The ancient opposed piston layout may come back and fight on.
The lighter, lesser friction surfaces and simpler design do not need the typical four-stroke engine valvetrain, intake and exhaust ports, and complexity of synchronized camshafts and valves.
Electronic control now allows the optimum metering of air and fuel precisely and could take advantage of the design's inherent simplicity and efficiencies.

"Simpler design" should have a disclaimer. It's interesting that you could possibly do away with camshafts, but without additional hardware you now have a fixed valve timing engine. I foresee most solutions to this being more complicated and potentially more failure prone than a balanced rotating shaft.

Plug-in hybrids (or EVs with "range extenders) are certainly one solution to battery limitations, but ICEs today are large and complicated, and take up a significant portion of a vehicle. Anything that can downsize the footprint without sacrificing efficiency would be great.

Modern snow mobiles run oil injection separate from the fuel.
The exhaust valve engines appeared to allow for more power at full throttle and delay the exhaust rejection to allow more time for better fuel burn at part throttle.

The part throttle or variable valve issues could be solved by teaming it with an electic propulsion and running the motor at max power just to recharge batteries. A setup like that should be allowed to have higher emissions while it's running because maybe it only needs to run 1/2 or 1/3 of the time.